The present invention generally relates to electrical connections for surface-mount circuit components of hybrid circuits. More particularly, this invention relates to a thick-film solder stop for a solder joint of a surface-mount component, in which the solder stop has a composition that promotes the thermal cycle fatigue resistance of the electrical connection.
Flip chips, ball grid arrays (BGAs), wire bond pads, chip resistors and chip capacitors are examples of surface-mount devices, i.e., discrete circuit devices mounted to the surface of a circuit board, such as a printed circuit board (PCB), ceramic substrate, printed wiring board (PWB), flexible circuit, or a silicon substrate. These devices rely on solder joints to both secure the chip to a circuit board and electrically interconnect the device to conductors formed on the circuit board. The size of a flip chip is generally on the order of a few millimeters per side, while bond pads, chip capacitors and resistors are typically smaller. As a result, the conductors required for surface-mount devices are narrow, e.g., line widths of about 0.5 millimeter or less, and typically spaced apart about 0.5 millimeter or less.
Because of the small size of the solder joints, soldering a surface-mount device to its conductor pattern requires a significant degree of precision. Reflow solder techniques are widely employed for this purpose, and typically entail precisely depositing a controlled quantity of solder using methods such as printing and electrodeposition. For smaller surface-mount devices, such as chip resistors and capacitors, the chip is soldered to its conductors by registering terminals formed on the chip with solder deposited on the conductors, and then reheating, or reflowing, the solder so as to form a xe2x80x9csolder columnxe2x80x9d that metallurgically adheres and electrically interconnects the chip to the conductors, yielding a solder joint. Mounting of flip chips and BGAs differ in that the solder is typically deposited on bond pads on the chip. Thereafter, the chip is heated above the liquidus temperature of the solder to yield xe2x80x9csolder bumps.xe2x80x9d After cooling to solidify the solder bumps, the chip is soldered to the conductor pattern by registering the solder bumps with their respective conductors and then reflowing the solder, again forming solder joints.
Placement of the chip and reflow of the solder must be precisely controlled not only to coincide with the spacing of the terminals and the size of the conductors, but also to control the orientation of smaller surface-mount devices and the height of flip chip solder joints after soldering. As is well known in the art, smaller chips are prone to twisting and tilting during reflow as a result of the device floating on the surface of the molten solder, while controlling the height of flip chip solder joints after reflow is often necessary to prevent the surface tension of the molten solder bumps from drawing the flip chip excessively close to the substrate during the reflow operation. Sufficient spacing between a flip chip and its substrate, which may be termed the xe2x80x9cstand-off height,xe2x80x9d is desirable for enabling stress relief during thermal cycles, allowing penetration of cleaning solutions for removing undesirable processing residues, and enabling the penetration of mechanical bonding and encapsulation materials between the chip and its substrate.
The position and height of a solder column of a discrete component are generally controlled by limiting the surface area over which the printed solder is allowed to reflow. As illustrated in FIG. 1, which shows a conductor 12 in longitudinal cross-section, the latter approach typically involves the use of a solder stop 14, which is typically formed by a solder mask or printed dielectric. The solder stop 14 extends widthwise across the surface 18 of the conductor 12, which is printed or otherwise formed on a dielectric substrate 10, such as alumina. A solder joint 16 is shown as joining a surface-mount (SM) component 20 to the surface 18 of the conductor 12, as would be the case after solder has been printed and reflowed on the conductor 12, and the component 20 then registered and reflow soldered to the conductor 12. As is apparent from FIG. 1, the solder stop 14 delineates an area on the surface 18 of the conductor 12 over which solder is able to flow during reflow to form the solder joint 16. By properly locating the solder stop 14 on the conductor 12, the degree to which the molten solder can spread during reflow is controlled, which in turn determines the height of the solder joint 16 and therefore the stand-off height of the component 20 relative to the substrate 10.
Because solder is registered and soldered directly to the conductor 12, the conductor 12 must be formed of a solderable material, which as used herein means that a tin, lead or indium-based alloy is able to adhere to the conductor 12 through the formation of a metallurgical bond. In contrast, the solder stop 14 is intentionally formed of a nonsolderable material, meaning that solder will not adhere to the material for failure to form a metallurgical bond. Upon reflow, the reflow area defined by the solder stop 14 on the conductor 12 causes the solder joint 16 to have a columnar shape between the component 20 and the conductor 12.
Though widely used in the art, trends in the industry have complicated the ability for solder stops to yield solder joints that exhibit adequate reliability. Particularly, the trend is toward the use of low-melting, high-tin (e.g., 60Snxe2x80x9440Pb) solder that is relatively brittle. Thermal cycle reliability problems can occur when a brittle solder solidifies against a solder stop used to contain the solder during reflow. During thermal cycling, fatigue fractures 22 tend to occur in the conductor 12 at the junction between the solder joint 16 and solder stop 14, as shown in FIG. 1. The cause of the fracture 22 is generally the mismatch of the coefficients of thermal expansion (CTE) of the conductor 12, solder stop 14 and solder. Solder stops are typically a hard thick-film dielectric material having a CTE roughly equal to that of the alumina substrate (about 6.7xc3x9710xe2x88x926/xc2x0 C.), while the CTE of the solder is typically much higherxe2x80x94e.g., about 25xc3x9710xe2x88x926/xc2x0 C. for lead-tin solders. The CTE mismatch is further exasperated by surface-mount components whose CTEs are typically about 4xc3x9710xe2x88x926/xc2x0 C. to about 25xc3x9710xe2x88x926/xc2x0 C. The resulting stresses developed in the joint during thermal cycling due to the large CTE mismatch are thought to be intensified or concentrated at the solder-conductor-solder stop interface, where the solder is inhibited by the hard solder stop material. Eventual stress relief is achieved through the creation of a crack through the underlying conductor, as shown in FIG. 1. The fracture occurs in the conductor because the conductor is the weakest structure at the solder-conductor-solder stop interface, and is therefore more susceptible to fracture by thermal cycle fatigue than the solder joint and solder stop.
To reduce the occurrence of fatigue fractures in conductors, solder stops formed of nonconductive polymer materials have been used that absorb the stresses created by the highly expanding solder. However, suitable polymers exhibit inferior printing characteristics and require special curing processes that are not conducive to thick-film manufacturing processes. Accordingly, it would be desirable if an improved solder stop were available that was capable of diverting or absorbing thermal stresses generated by the CTE mismatch at the conductor-solder stop interface.
It is an object of this invention to provide an electrical connection comprising a conductor, a solder joint, and a solder stop that inhibits fracturing of the conductor and solder joint during thermal cycling.
It is another object of this invention that the solder stop has a composition that renders it more susceptible to fracturing by thermal cycle fatigue than the conductor and solder joint.
It is yet another object of this invention that the solder stop sacrificially fractures during thermal cycling to relieve thermal stresses in the electrical connection, and therefore promotes the reliability and durability of the electrical connection when a surface-mount circuit device is bonded to the conductor with the solder joint.
The present invention provides an electrical connection for a surface-mount circuit device, and a method for forming the electrical connection. The electrical connection includes a solder stop that promotes the accurate location of a solder joint that electrically connects the surface-mount device to the conductor. In accordance with this invention, the solder stop also promotes stress relief of the electrical connection during thermal cycling, such that thermal cycle fatigue cracking occurs in the solder stop instead of the conductor and solder joint. As a result, thermal stresses are absorbed and dissipated by the solder stop, and do not adversely affect the continuity and mechanical integrity of the electrical connection.
According to this invention, the above benefits are achieved by the solder stop having a composition that renders it more susceptible to fracture by thermal cycle fatigue than the conductor and solder joint. To achieve this characteristic, the solder stop contains an inorganic particulate filler in a glass matrix, the latter being present in an amount that forms a weak bond between the inorganic particles and between the solder stop and conductor. The inorganic filler can be one or more of the following oxides: zirconia, alumina, silica, barium oxide, calcium oxide, magnesia and lanthana. The glass matrix can be formed by one or more of the following oxides: calcium oxide, alumina, lead oxide, boric oxide and silica. The inorganic filler preferably constitutes at least 75 weight percent of the solder stop, with the balance being the glass matrix and possible additional adjuncts, including inorganic colorants. The solder stop is preferably a thick-film dielectric formed by firing a thick-film ink that contains, by weight, about 20% to about 40% of an organic vehicle, up to about 5% of an inorganic colorant, with the balance being a mixture of the inorganic particulate filler and glass frit.
A solder stop having the above-described composition is generally porous, and therefore has multiple crack initiation sites. In addition, because the solder stop contains only enough glass matrix material to form a weak bond between the inorganic particles and between the solder stop and conductor, stresses induced in the electrical connection will eventually produce multiple cracks in the solder stop adjacent the solder joint instead of cracks in the conductor or solder joint. In effect, the solder stop is a sacrificial nonconducting member of the electrical connection, in which thermal stresses are concentrated and fatigue cracks are encouraged to develop in order to relieve thermal stresses in the electrical connection. Consequently, continuity and mechanical integrity of the conductor and solder joint, and therefore the electrical connection, are maintained.
Other objects and advantages of this invention will be better appreciated from the following detailed description.